The prosecution is trying to prove beyond reasonable doubt Pistorius killed her intentionally

Oscar Pistorius faced another day of relentless cross-examination Friday as the prosecution challenged his account of the killing of his girlfriend, Reeva Steenkamp.

Prosecutor Gerrie Nel has accused the athlete of hiding the truth about the death of Steenkamp, whom he shot last year through a closed toilet door in his home in Pretoria, South Africa.

His questions again sought to undermine Pistorius' reliability and credibility and to portray the Olympic athlete as someone who was inventing his version of events and "tailoring" evidence to suit his story.

As Nel turned once again to the early hours of Valentine's Day 2013, he repeatedly challenged Pistorius over his actions in the moments leading up to Steenkamp's death.

The prosecution's argument is that Pistorius shot Steenkamp intentionally after a heated argument. Pistorius does not deny shooting her but insists that he mistook her for an intruder.

Pistorius said he thought he heard the toilet door opening before he fired.

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"I didn't intend to shoot. My firearm was pointed at the door because that's where I believed that somebody was," he said. "When I heard a noise, I didn't have to think, and I fired -- I fired my weapon. It was an accident."

Nel, known in South African legal circles for his bulldog-like approach to cross-examination, responded to Pistorius' testimony almost with scorn.

"Your version is so improbable that nobody would ever think that it was reasonably, possibly true," he said.

Nel then hammered Pistorius on whether he had known Steenkamp was in the toilet when he fired.

"You knew Reeva was behind the door, and you shot at her," Nel said more than once.

"That's not true," Pistorius replied in a low tone.

That dramatic moment was when Nel asked for the trial to be adjourned until Monday morning.

Prosecutor: You wanted to shoot

Pistorius earlier denied being "ready to shoot" as he made his way to the bathroom where he says he heard what he thought was an intruder.

But he agreed that he had taken off the safety catch so he could fire if needed. "I didn't want to take anybody's life. I screamed for the intruders to get out of my home," he said.

"You wanted to shoot," contended Nel, who on Thursday sought to build a picture of Pistorius as an arrogant hothead who is reckless with guns.

Pistorius replied that there is a "massive difference" between being ready for something and wanting to do it.

Asked by the prosecutor why he approached the apparent threat rather than seeking to move out of harm's way, the athlete said it was his in his nature to respond that way.

"I wanted to put myself between the perceived danger and Reeva," he said. "I wish I did all these other things put to me."

Nel also argued that it was "not possible" that Steenkamp would not have responded when Pistorius screamed to her about what he thought was an intruder in the house, as he has said happened.

Steenkamp was only 3 meters from Pistorius at the time, behind the toilet door, the prosecutor said.

"She would've been terrified, but I don't think that would've led her to call out," Pistorius said, arguing that Steenkamp would have assumed the danger was getting closer.

"She wasn't scared of anything except you. She wasn't scared of an intruder. She was scared of you," replied Nel.

The prosecutor also pressed Pistorius over whether he heard a woman screaming during the shots he fired, as some witnesses have said they heard. Pistorius said he did not.

Pistorius: I was fixated on the threat

As Nel went through the events leading up to the point of the shooting, Pistorius said that after getting up to close a balcony door and move fans inside, he heard the bathroom window sliding open and slamming into the frame.

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Nel repeatedly asked him why he hadn't at that point asked Steenkamp whether she too had heard the noise.

Pistorius replied that he didn't because he was sure about what he had heard. He said he whispered to Steenkamp to get down and call police.

Nel asked whether he had waited for a response, as he said would have been reasonable, pointing out that his ex-girlfriend Samantha Taylor had testified he had done that on a previous occasion when he'd heard a noise.

"I never waited for a response. ... My whole body was fixated on the threat," Pistorius answered.

Nel's questions then focused on the position of certain items in the bedroom, including a duvet, the fans and a pair of jeans, all of which Pistorius says were moved by police. Nel sought to argue that they do not support Pistorius' version of events.

At one point, the judge reprimanded Nel for calling Pistorius a liar and told him to mind his language.

More than once, Nel suggested that Pistorius had difficulty remembering what happened because he had made things up.

Pistorius became emotional as the cross-examination continued, prompting Nel to ask him why.

"This is the night I lost the person that I cared about. I don't know why people don't understand that," Pistorius responded.

As he broke down in tears, the judge ordered a short break to allow him time to gather himself.

Nel has previously accused Pistorius of becoming emotional when the questions get difficult.

Pistorius quizzed about mistakes

During cross-examination earlier Friday, Pistorius made mistakes in answering questions about repair work and his alarm system.

He attributed the mistakes to fatigue, prompting Nel to ask whether he was too tired to continue in the stand.

After Judge Thokozile Masipa pressed Pistorius on the question, the athlete said he wasn't making mistakes because he was tired -- prompting Nel to ask why, in that case, he was making mistakes.

A little later, Nel made a mistake while questioning Pistorius, who pointed it out. The prosecutor said Pistorius wasn't too tired to highlight the mistakes the prosecutor himself was making in his questioning.

Nel also focused on security on the gated Silverwoods estate where Pistorius lived, pointing out that despite his claimed fear of burglary, the athlete left his cars outside and had not immediately fixed a broken window in his house.

Pistorius' message exchanges with Steenkamp

A day earlier, the athlete denied that he acted selfishly toward Steenkamp.

In a bid to paint their relationship as rocky, he ripped apart message exchanges between the couple Thursday.

Nel also sought to paint him as selfish and demanded to know why the athlete did not respond to his girlfriend's declaration of love.

But Pistorius said he preferred to talk to his girlfriend over the phone rather than messaging. He acknowledged he never got a chance to tell her that he loved her.

"Because it was all about Mr. Pistorius," Nel said.

The runner has admitted to the killing but said he mistook Steenkamp for an intruder in the bathroom when he fired through the toilet door and killed her.

The prosecution alleges that Pistorius killed his girlfriend after they argued. Several witnesses have testified to hearing a man's shouts coming from the house, although they have also spoken of the terrified screams of a woman leading up to and during a volley of shots.

The trial has gripped South Africa, where Pistorius is considered a symbol of triumph over physical adversity. His disabled lower legs were amputated when he was a baby, but he went on to achieve global fame as the "Blade Runner," winning numerous Paralympic gold medals on the steel blades fitted to his prostheses.

Only those in the courtroom can see Pistorius because he has chosen not to testify on camera. His testimony can be heard on an audio feed.

The trial is scheduled to continue until the middle of May.

Masipa will decide the verdict in collaboration with two experts called assessors. South Africa does not have jury trials.